Membranes are usually defined as thin walled structures having an open spongy morphology with a limited pore size distribution. The preferred membranes are microporous membranes, that is, membranes having a typical pore size range between approximately 0.01 .mu.m and 10 .mu.m; although it is to be understood that the principles of the present invention are applicable to membranes having a still greater range of porosities. Traditionally, microporous membranes are useful to remove fine contaminants from liquid and gas media. The particulate material, such as dust or bacteria, can be removed by a microporous membrane filter through the mechanical sieving mechanism. In this mechanism, the filtration efficiency is controlled by the relative size of the particulate and membrane pore size. To achieve total particle retention, a microporous membrane with fine pore size is required to meet the requirement of the pharmaceutical and electronic industries. However, membranes with such a fine pore size tend to have undesirable characteristics of high pressure drop across the membrane, lower flow rate, and shortened membrane life. The suspended particulate materials can also be removed from the fluid by the electrokinetic adsorption mechanism. Most suspended particulate materials which are commonly encountered in industrial practice have a negative zeta potential. By imparting a positive zeta potential to the surface of filter membranes, the particulates capture efficiency can be greatly improved. This is true even for particulates whose size is much smaller than the membrane pore size. As a result, a membrane with charge modification has a lower pressure drop, a better fluid flow rate, and a longer life than a membrane filter with a comparable pore size rating but without any charge modification.
Conventional cationic charge modified microporous membranes for the filtration of ultrapure water typically have a proper charge density, but a slow 18 megaohm-cm water resistivity recovery, a characteristic critical to filtration application in the electronic industry. U.S. Pat. No. 4,702,840 discloses charge modified membranes prepared by casting an acidic polymer solution comprising a matrix polymer and a primary activated polymer having epoxy functional groups on a supportive material to form a thin film, followed by immersing the film in a liquid bath. The resulting nascent membranes are washed in water and finally dried in an oven. During this membrane fabrication process, epoxy functional groups of the primary activated polymer available for the reaction with amino or carboxyl functional groups of the polyamide matrix polymer will be de-activated through an acid-catalyzed ring-opening reaction due to the presence of an acidic solvent. Therefore, complete grafting of the primary activated polymer on the polyamide membrane surface can be a problem.
It is an object of the present invention to provide charge modified microporous membranes for filtration applications, particularly the filtration of ultrapure water used in the manufacture of computer chips in the electronic industry where a fast 18 megaohm-cm water resistivity recovery is an important requirement.